The present disclosure is directed to a discharge system which forms photons which are emitted from a pulsed spark discharge. The photon spectra are primarily a broad band source of excitation. The spark interacts with a flowing helium stream to thereby provide the necessary photon emission. For this system, the apparatus utilizes a pair of spaced electrodes which provide a transverse spark across the gap between the electrodes so that the photon emission occurs.
One aspect of the present invention is the provision of a pulsed discharge system using periodically formed pulses. This enables better control of the discharge, and avoids the formation of an ionic, radiant cloud which is typical with an RF discharge system. The source is therefore discrete, fixed and finite while the RF system provides a diffused energized region. It is therefore one advantage of the present system which is uniquely able to provide the necessary ionic conversion while serving as a detector in analytical chemistry.
As mentioned in the :referenced patent above, the use of a spark discharge has advantages in that it is a different type of discharge in comparison with RF discharge. In particular, it is able to provide a discharge which is sharply defined. One aspect of this relates to the use of the present apparatus to form a spark of fixed, relatively short duration so that the exposed gas flow interacting with the spark which is held to a controlled minimum time duration. This limits the dissipation of energy from the spark. In this particular aspect, it appears that the energy discharge is primarily through photon emission. Thus the photon emission comes from a single region, a carefully defined and geometrical constrained region. A carrier gas as will be discussed is delivered through the region for interaction thereby enabling the emission of energy by means of photon discharge which interacts to provide a detector system.
One aspect of the present disclosure relates to the duty cycle. The duty cycle is relatively brief. For example, the present system can be used with a pulsed DC source to emit a narrow, sharply defined spark of relatively brief duration. This narrow spark is able to be replicated over a long interval without damaging or fatiguing the equipment. Since the equipment is primarily off because of the low duty cycle implemented, it operates at a relatively reduced temperature and is less subject to spark originated damage. The damage which typically occurs includes contact or electrode tip pitting. It also includes the unintended deposition of vaporized materials on the internal surfaces of the equipment which changes the nature of those surfaces. Those surfaces can become unintended impurity traps. By utilization of the reduced duty cycle which is set forth in this apparatus, the fatigue of the equipment is materially reduced. In one aspect of the present system the ionization system utilizes a pulsed DC spark which is applied from one electrode to another. With a brief duty cycle, the repetition rate can be in the audio frequency range. The pulses can occur as rapidly as perhaps 1000 to 2000 pulses per second. Because each pulse is separate and is relatively isolated, the pulses are able to be formed from one electrode to the other in a monopolar sequence, or they can alternately be formed in a bipolar sequence of separate monopolar pulses. This is a matter of convenience in the arrangement of the pulse forming circuitry. Thus, alternating pulses can have opposite polarity without making more complicated electronic circuitry which provides the power supply. Even though they alternate in polarity, the system nevertheless remains a DC pulse system because each pulse forms its own spark and photon discharge which is extinguished, and the following pulse is formed without interplay between consecutive pulses.
One important aspect of this system is the injection of the gas flow to be tested remote from the spark discharge. It is typically used as an output device for use with a chromatographic column, typically with a gaschromatograph (GC) column. The output flow of the GC column includes a carrier or other solvent. A typical GC column comprises a mobile phase and a stationary phase. The mobile phase comprises a carrier gas or solvent into which sample gas containing one or more sample compounds are injected. The stationary phase comprises one or more solid constituents within the GC column which exhibit different retention times for the "unknown" sample compounds. The sample gas containing unknown compounds is injected over a relatively short period of time into the carrier gas flow near the input of the GC column. Sample compounds are retained for different times by the stationary element of the GC, and then subsequently released. Upon release, each type of sample compound is swept by the carrier gas from the GC column and discharged in the form of a "peak" or maxima in concentration in the carrier gas. Retention times, and therefore time separation of the unknown sample peaks, is a function of several factors including the carrier gas flow rate and the type of the stationary phase within the GC column. The injection, near the input of a GC column, of sample gas containing multiple compounds results in the subsequent release, or "eluates", of maxima or peak concentrations of individual compounds at the output of the GC column. The GC separates sample compounds by eluting in the form of concentration maxima or peaks in the output carrier gas at varying times, measured from the injection of the composite sample gas. As described, the GC process does not quantify the concentrations of the sample compounds, but does separate multiple compounds for further analysis using the detector disclosed herein. By using a series of calibration gases, a fixed flow rate, and a specific fixed phase material, the GC process can be used to identify compound types based upon the time position of the eluted peaks, measured with respect to the injection of the composite gaseous sample. The various peaks which are eluted through operation of the column in conjunction with the solvent or carrier may provide a sample which, in the immediate region of the spark, would form unintended soot deposits, ash or residue on the surfaces of the system. In this instance, the sample material is delivered for testing downstream of the spark but in view of the spark so that the spark is able to generate photon irradiation for the sample material delivered from the GC column or other suitable source. The output of the GC column or other test instrument is typically input into a region immediately adjacent to but offset from the spark source so that the spark does not consume the sample material in operation of the system.
Downstream from the spark source, the system utilizes first and second terminals. Two terminals are spaced on a support or mounting structure and are equipped with rings which are exposed to the downstream gas flow from the spark. The flow in this region is preferably helium which is the preferred gas introduced into the system. There is an input port for helium delivered directly through into the spark gap. Some of the helium interacts with the spark during the spark while some of the helium flows through the spark gap while no spark is being formed. The flow of helium sweeps the area, thereby directing helium flow to the two electrodes. Because helium is inert, there is no interaction from the helium flow.
The GC solvent and the eluted constituents in that flow are delivered through a relatively small, centralized, counter flow line. The flow rate of the helium is greater by perhaps 10-30 fold. The flow rate of the helium delivered into the system is able to sweep the test instrument and eluted sample flow towards the outlet. The test instrument solvent is delivered through the counter flow tube which is advanced axially in the equipment to a location to obtain optimum interaction and signal measurement. More importantly, this interacts with the flow so that the helium in the system is able to provide the necessary photon emission with relatively broad spectra thereby assuring suitable interaction of the sample material to be measured. The broad sequence spectra of the emission helps assure that the test instrument is able to see the particular sample. Otherwise, there will always be the risk that irradiation would not occur because the sample would either be transparent to the radiation or the carrier gases (helium in the preferred form) would be opaque to the emission. The ability of the helium to deliver the radiation from the broad frequency spectra is an important aspect.
It is proposed as a theory or explanation of the operation of the present system that helium introduced into the spark gap is caused to emit photons as a result of temporary formation into an unstable diatomic bond. While most gases naturally formed diatomic molecules, helium is inert which means that it has no unsatisfied valance bonds available and therefore is normally an monatomic molecule. One explanation for the phenomena obtained in this system is that the helium is excited sufficiently that a fleeting momentary bond forms molecules of diatomic helium. In breaking down to the normal monatomic form of helium, the diatomic molecule emits photons in a relatively broad energy spectra. This has been demonstrated experimentally by simply imposing a temporary shutter so that the emitted light cannot be received downstream. In the reaction area, so long as the test instrument gas discharge is irradiated with the emitted spectra, detection does occur. With the insertion of a light blocking shutter, no interaction occurs, lending proof that the reaction involves the emitted photon radiation.
In another aspect of the present system, the region between the detector electrodes at which the test instrument solvent and eluted sample materials are introduced is determined by repositioning of a moveable tube. This moveable tube is moved so that sensitivity is optimized. Moreover, the optimum sensitivity that is obtained by the system enables one to assure that tuning for a particular test instrument source can be accomplished easily. Adjustment typically occurs only with modest movement. The specific location is thus adjusted and the injection tube is then locked in place.
One further aspect of the present system is its ability to provide a relatively sensitive output signal. For a given peak amplitude input to the instrument, an output voltage peak is obtained from the system by means an electrometer. The electrometer is connected across the electrodes so that the detected current flow can be used.
Summarizing, the present apparatus is a system which utilizes a pair of spaced electrodes terminating at electrode tips or faces which are spaced across a flow path. Helium is introduced in the flow path to flow through the region. Generally, the signal is off but when a pulse is formed, the current flow from electrode face to face occurs through the helium, thereby creating the photon irradiation mentioned. A short pulse is used and the pulse is extinguished, thereby terminating further formation of photon irradiation from operation. The helium flow is directed away from the two electrodes through an elongate tubular passage which houses two exposed rings. The rings serve as first and second electrodes for an electrometer output instrument. A coaxial introduction tube is inserted from the opposite end. It discharges at the tip end of the tube any solvent and eluted flow necessary for testing. The discharge from this tube includes the solvent and eluted unknowns delivered from the test instrument. On exposure to photon irradiation generated by the spark, the specimen or sample interacts with the photon spectra thereby forming charged particles which vary the electrometer output signal.